1. Field of the Invention
This invention generally relates to the measurement of temperatures and more particularly, relates to heat shields employed in instruments used in the measurement of high temperatures of gases such as those encountered in glass melting furnaces.
2. Description of the Prior Art
It is desirable in the glass making industry to measure accurately the temperatures of the gases at different locations in the overall system of producing glass, particularly the gas temperatures in a melting furnace occuring at its ports, regenerators tunnels, flues, chimney and ejectors. Measured under actual operating conditions, the temperatures serve as bases for improvements in operations, modifications in design, and fuel and power savings. As a result, longer furnace life, higher tonnages, improved quality, and lower costs for the production of glass may be achieved.
In a conventional glass melting tank furnace, fuel is alternately fired, using preheated combustion air, from one side and then the other through a series of ports along each side of the tank at right angles to the flow of molten glass. The raw materials are continually fed at one end of the tank and molten glass is removed from its other end. The variations and conditions at the various ports down each side of the tank are therefore important in determining the variations in temperature undergone by the raw materials during melting and the glass after melting.
In such glass melting furnace systems, the temperature of the combustion air and exhaust gases may be substantially different from the temperature of the surrounding bodies, and the heat exchanged by radiation between the bodies and the temperature measuring instrument may predominate over that exchanged by convection. The exchange of heat by radiation from or to the adjacent bodies can influence the instrument reading so that it may indicate the temperature of such bodies or some temperature intermediate that of the bodies and the combustion air or exhaust gases rather than the true temperature of these gases.
Generally, in the above-mentioned environment a sheathed thermocouple is employed to measure the temperature of a gas. As is known, a thermocouple indicates its own temperature and if it is to determine that of a gas, its hot junction must attain the temperature of the gas. In the case of the sheathed thermocouple, the surface of the sheath receives heat from the gas by convection. This heat then passes through the sheath to the hot junction of the thermocouple, but at the same time the sheath exchanges heat by radiation with the surrounding bodies and loses heat by conduction therealong. Hence, the temperature reached by the hot junction of the thermocouple may be different from the true temperature of the gas, particularly when temperatures above 1400.degree. C. (2550.degree. F.) are encountered.
Thus, in order to measure accurately temperatures of hot gases whose temperatures are different from those of their surroundings, aspirating pyrometers are conventionally employed. As is known, as aspirating pyrometer is an instrument wherein the convective heat transfer to a sheathed thermocouple from a gas is increased by drawing the gas over it at high velocity and at the same time shielding the thermocouple from heat radiating to or from the surrounding bodies so that the temperature of the hot junction of the thermocouple will be substantially the same as the temperature of the gas it is sensing.
In the past it has been proposed to construct a heat shield from standard thin wall refractory tubing. Generally speaking, this type of shield construction comprises a large diameter tube housing a series of small diameter tubes which are circumferentially arranged around and bonded to the inside surface of the large tube. In assembling this type of radiation shield, it is important that the small tubes hold the same relative position both with respect to each other and the large tube so that the transmission of heat to or from the sheathed thermocouple by the gases being sensed remains uniform. In the past, this has been accomplished by filling in the spaces defined by the inside surface of the large tube and the outside surface of the small tubes with a refractory cement. Thus, this form of construction merely provides a simple system consisting of, in effect, a refractory block having a central passage surrounded by a single series of parallel passages. Although bonding may hopefully be improved, it is at the expense of reduced pyrometer efficiency because passage of gas through the passages formed by the outside surface of the small tubes in conjunction with the inside surface of the large tubes is blocked. Too frequently, in this shield construction, the cement bond fails and the small tubes are drawn longitudinally from their original positions by action of the induced gas flow. This results in the disadvantages of (1) loss of calibration of the thermocouple, (2) possible loss of the thermocouple sheath and even the thermocouple junction, (3) loss of time in repeating the masurements, (4) costly loss of the heat shield and (5) loss of time in dismounting and remounting a new heat shield to a probe.